1. Field Of The Invention
The present invention relates generally to heavy duty machinery used in the mining industry and, more particularly, to a system for controlling the feed rate of a vibrating feeder used in the processing of bulk materials that have been excavated or blasted from pits, quarries and mines. The system is designed to continuously monitor the feed rate of the materials being discharged from the feeder and automatically adjust the slope angle of the feeder to maintain a controlled feed rate which will not exceed the load capacity of downstream conveyor units and other secondary machinery and will prevent unwanted surges that can cause damage to these same units. The invention is also directed to a particular process for controlling the feed rate of a vibrating feeder.
2. Description of the Prior Art
For years, the mining industry has been utilizing both stationary and mobile in-pit processing plants which are capable of on-site processing of bulk materials that have been excavated or blasted from the working face of a quarry, pit or mine. The mobile plants are particularly useful since they are designed to be readily transported within the pit or quarry to follow the progress of the mining operation. The mobility of these plants also permit them to be transported, when necessary, from one job site to another.
A typical mobile plant is usually made up of individual stations which provide a number of different processing operations. Generally, primary crushing or reduction of the bulk materials is the primary concern. For this reason, a primary crushing station which includes heavy duty machinery for crushing larger size rocks into a more manageable size is utilized. The materials coming from the primary crushing station can be transported via conveyor units to secondary stations where further processing can take place. The secondary stations can, for example, further reduce the size of the materials, screen out dirt and fines, separate particular size products, and wash the materials.
The primary crushing station is an important unit on any mining site since it handles the first stage of material reduction. A typical primary crushing station includes a vibrating feeder, a hopper, rock crushing machinery and a take-away conveyor unit. The vibrating feeder includes a carrier pan or trough which handles the materials and feeds the conveyor unit and rock crushing machinery. A hopper is usually placed above the feeder to allow the bulk materials to accumulate and then drop down onto a load receiving region of the pan where vibratory energy imparted to the pan by a motor or other actuating device causes the materials to move along the length of the pan to a discharge region. The pan is usually sloped at a particular angle with the horizontal, called the "slope angle" of the feeder, to allow gravity to help facilitate movement of the materials. The rate at which the materials move along the pan for discharge from the feeder depends upon the amplitude and frequency of the vibration and the particular slope angle of the feeder.
The vibrating feeder may also include a section added to the pan surface that is made from a number of parallel, spaced-apart bars. This section, called a "grizzly" section in the industry, allows under size rocks and materials to fall through the spaces between the bars to a chute that directs the materials to the take-away conveyor located beneath the feeder. Larger size rocks that cannot fall through the spaces between the bars are fed into the rock crushing machinery to reduce their size. The crushed rocks can then be fed onto the take-away conveyor which would also be located near the crushing machinery.
While the primary crushing station operates somewhat efficiently, there are disadvantages associated with it which affects the overall performance and productivity of the unit. The problem stems from the fact that the raw materials that are collected by the front end loaders, quarry trucks, or pit shovels at the work site are heterogeneous, i.e., a combination of materials ranging from fine rock and dirt to large, coarse rocks. The speed at which the raw materials can be processed by the vibrating feeder can vary greatly depending upon the composition of the raw materials and the manner in which the hopper is loaded. For this reason, it is not uncommon for the take-away conveyor or secondary conveyors that transport the materials from the primary crushing station to be overloaded with materials at times and later contain little or no materials.
An uneven feed rate on the take-away conveyor is detrimental to the power source of the conveyor system since it can create surges which strain the power source. An uneven feed rate can also cause the materials to overflow from the conveyor. As a result, conveyor systems have been over designed or oversized to meet the surge possibilities which ultimately results in increased manufacturing and maintenance costs for the conveyor systems. Since long conveyor systems are now being used with primary crushing stations due to the advances in conveyor design and the cost effectiveness of transporting the materials via conveyors rather than by trucks, the overall costs resulting from the over designing of the conveyor systems can be quite high.
One way to effect a more uniform feed rate from the vibrating feeder and reduce the costs attributable to the over design of the conveyor systems is to maintain an operator at the primary crushing station. The operator can control the feed rate to some extent by constantly stopping and starting the machinery depending upon the amount and size of the raw materials that are being loaded into the primary crushing station. The operator can also adjust the feed rate by controlling the amount of material being placed into the hopper. For example, the operator can signal the load operator by a horn, light or other signaling device whenever the material flow to the hopper has to be increased or decreased. However, even an experienced operator can cause uneven feed rates through inattention or inability to estimate the weight of the material being fed onto the take-away conveyor or into the hopper. Further, the manpower needed to continuously monitor the primary crushing station is not cheap and adds to the cost of the mining operations.
The mining industry presently favors a vibrating feeder which utilizes a rotating eccentric weight to generate the vibrations to the feeder pan. This type of feeder is preferred at primary crushing stations due to its ruggedness, longevity, freedom from leakage, low profile, first cost and long term cost effectiveness.
These particular vibrating feeders operate at low frequencies which is most effective for feeding larger stones. The feeder usually operates at a normal operating frequency that is generally above the first natural harmonic. Attempts have been made to create a control system for such feeders which will allow an operator to adjust the speed of the feeder to vary the amplitude or frequency of the vibrations in order to control the feed rate. However, such control systems have not been particularly successful due to the fact that a change in the speed of the feeder may place it at one or more of its natural or harmonic frequencies which is detrimental to the machinery. For the most part, these particular feeders operate best at an optimum speed which is maintained throughout the daily operation. Any control of the feed rate must generally be accomplished by controlling the amount of materials being fed into the feeder hopper or by starting and stopping the feeder. Other ways of controlling the feed rate such as adjusting the eccentric weight to the change the amplitude of the vibration or varying the slope angle of the feeder are not viable alternatives during operation. Once the slope angle and the eccentric weight have been set on the feeder, they cannot be changed once the feeder is operating.
As a result of the disadvantages associated with the rotating eccentric weight type feeders, there is a need for a system for controlling and varying the feed rate of the feeder which still enables it to work at its optimum speed. Therefore, there is a need for a control system that will eliminate all of the above disadvantages associated with this type of vibrating feeder and which will achieve a feed rate that is not detrimental to secondary handling systems.